A circuit substrate is formed using a prepreg obtained by impregnating a sheet-shaped base member, such as a glass fiber base material, with thermosetting resin. For example, Japanese Patent Application Laid-open No. 2004-216784 (hereinafter, simply referred to as “Patent Document 1”) discloses a method for manufacturing a prepreg by immersing a glass fiber base material having a thickness of about 50 to 200 μm into a thermosetting resin varnish.
In a prepreg obtained by such a method, a resin composition is supported by the glass fiber base material, and two resin layers formed of the resin composition are symmetrically provided on both surfaces (sides) of the glass fiber base material. More specifically, such a prepreg has a structure in which two resin layers formed of the same resin composition and having the same thickness are provided on both the surfaces of the glass fiber base material.
Meanwhile, in recent years, electronic parts, electronic devices and the like are becoming increasingly smaller and thinner, and therefore circuit substrates and the like used for them are also required to be smaller and thinner. This leads to necessity for forming a higher-density circuit wiring portion (circuit wiring pattern) in the circuit substrates.
In order to form such a higher-density circuit wiring portion, a multilayer circuit substrate is used. In addition, various attempts have been made to reduce a thickness of each layer of the multilayer circuit substrate.
From a viewpoint of redaction of a thickness of a multilayer circuit substrate, generally, such a multilayer circuit substrate is manufactured by preparing a plurality of prepregs each having a circuit wiring portion on one surface thereof, and laminating the prepregs so that the circuit wiring portion provided on the one surface of the prepreg is embedded into an opposite surface of another prepreg laminated on the prepreg.
In this case, the one surface of the prepreg, on which the circuit wiring portion is to be formed, is required to have plating adhesiveness, and the other surface of the prepreg, into which a circuit wiring portion of the other prepreg is to be embedded, is required to have embeddability (moldability) for filling gaps between wires constituting the circuit wiring portion.
However, as described above, the prepreg disclosed in Patent Document 1 has two resin layers formed of the same resin composition on both the surfaces of the base material. Therefore, it is difficult to select a resin composition which allows each resin layer to have both properties of the plating adhesiveness and the embeddability. Such a problem becomes particularly conspicuous in the case where an attempt is made to reduce the thickness of the prepreg.
Further, there is also a case that a circuit wiring portion is embedded into each of two resin layers of a prepreg.
However, in the case where two circuit wiring portions having a different size such as thickness are embedded into the prepreg disclosed in Patent Document 1, there is a case that an amount of the resin composition constituting the resin layers becomes larger or smaller than a required amount of a resin composition to be filled in gaps between wires constituting each circuit wiring portion (hereinafter, also simply referred to as “gaps in a circuit wiring portion”).
As a result, there is a problem in that the resin composition is squeezed out of an obtained substrate from a side surface thereof, the circuit wiring portion cannot be reliably embedded into the resin layer or the like.
As described above, it is difficult for the prepreg disclosed in Patent Document 1 to satisfy the following two requirements: (A) the prepreg has both properties of the plating adhesiveness and the embeddability; and (B) an amount of a resin composition can be set appropriately depending on a circuit wiring portion to be embedded thereinto.
Further, it is also difficult to manufacture a prepreg using a thin glass fiber base material by a conventional method.